1. Field of the Invention
The present invention relates to the purification of phenolic esters of aromatic carboxylic acids and more particularly to the purification of diphenyl esters of aromatic carboxylic acids, such as phthalic acids, to provide ester monomers having good color properties.
2. Description of the Prior Art
The production of various phenolic esters of aromatic benzenedicarboxylic acids, such as diphenyl terephthalate, have become of significant commercial interest in recent years due to their use in a great many types of processes. As an example, diphenyl terephthalate and diphenyl isophthalate when dissolved in a solvent may be reacted with a primary diamine to produce polyamides. Likewise, 3,3'-diaminobenzidine may be condensed with various diphenyl esters to form polybenzimidazoles. In the synthesis of polyarylate engineering resins, such as Durel.RTM., a mixture of iso- and terephthalates is reacted with bisphenol A to provide resins which are of significant commercial interest. These esters may be produced by reaction of acid chlorides with a phenol to produce the phenyl ester and hydrogen chloride as a by-product, or by reacting a phenolic compound and aromatic dicarboxylic acid in the presence of a catalyst consisting of an alkali metal compound and a boron compound. While such processes are effective for producing phenolic esters they suffer the disadvantage that during the process of esterification, the ester acquires a pink to dark brown color which present methods of purification, such as vacuum distillation, recrystallization and/or carbon treatment, are insufficient to remove. Accordingly, there is a need for additional purification methods which are sufficient to achieve specification grade diphenyl phthalates.
U.S. Pat. No. 2,780,643 discloses subjecting synthetic alcohols to hydrogenation for color improvement followed by esterification of the alcohol with phthalic acid. U.S. Pat. No. 3,487,100 discloses purification of crude bis-B-hydroxyethyl terephthalate (BHET) for color improvement by reducing an aqueous solution of BHET in the presence of hydrogen and a hydrogenation catalyst. U.S. Pat. No. 4,076,946 discloses a process in which molten dimethyl terephthalate containing aldehydic impurities is subject to catalytic hydrogenation, without significant ring hydrogenation, to reduce the aldehydic impurities to compounds which can be tolerated in fiber grade polyethylene terephthalate.